1. Field of the Invention
The present invention generally relates to an optical information recording medium, an optical information recording/reproducing system, and an optical information recording/reproducing apparatus.
2. Description of the Related Art
In recent years, with the development of digital technologies and the improvement in data compression techniques, optical disks such as a digital versatile disk (DVD) have gotten a lot of attention as media for recording information such as music, movies, photographs, and computer programs. Also, as the prices of optical disks become lower, optical drives for recording and/or reproducing information on optical disks have become widespread.
Meanwhile, the data sizes of contents are increasing year by year and there is an increasing demand for optical disks with higher storage capacity. One way to increase the storage capacity of an optical disk is to provide multiple recording layers. Currently, development of optical disks having multiple recording layers (hereafter, may also be called “multilayer disks” or “multilayer optical disks”) and optical drives for recording and/or reproducing information on such multilayer disks is very active.
However, if the number of recording layers in a conventional optical disk is increased, the amount of light reflected from a recording layer decreases as the distance between the recording layer and the incidence plane increases, because light is absorbed by other recording layers. If the amount of light reflected from a distant recording layer decreases much, it becomes difficult to detect a signal. Also, a conventional laser diode may not be powerful enough to record information on such a multilayer optical disk. These problems have been limiting the number of recording layers in an optical disk.
To solve the above problems and thereby to increase the number of recording layers, multilayer disks using two-photon absorption materials have been proposed. The refractive index of a two-photon absorption material changes when it absorbs two photons simultaneously. The proposed multilayer disks utilize this characteristic of two-photon absorption materials. On a multilayer disk composed of a two-photon absorption material, information is recorded by changing the refractive index of target areas. These refractive index changed areas are called pits. More specifically, information is represented by the lengths and combination of refractive index changed areas and refractive index unchanged areas.
The probability of occurrence of two-photon absorption is proportional to the square of an applied optical-electric field (intensity of incident light). Therefore, two-photon absorption occurs only in an area where the energy of incident light is concentrated. When incident light is focused by a lens, two-photon absorption occurs only around the focal point and does not occur in other areas where the incident light is not focused. In other words, the refractive indices of recording layers away from the focal point do not change and those recording layers transmit the incident light without changing its intensity. Therefore, if a two-photon absorption material is used for recording layers, increasing the number of recording layers does not make it difficult to detect a signal or cause recording power shortage problems.
Thus, using two-photon absorption materials makes it possible to increase the number of recording layers and thereby to greatly increase the storage capacity of an optical disk. However, as in the case of conventional multilayer disks, forming guide tracks on each of the recording layers results in increased costs.
To obviate this problem, multilayer disks having guide tracks on a layer(s) other than recording layers have been proposed (see, for example, patent documents 1 and 2).
[Patent document 1] Japanese Patent Application Publication No. 2002-312958
[Patent document 2] Japanese Patent Application Publication No. 2005-18852
Patent document 1 discloses a recording medium where recording layers are provided only on one side of a guide track layer. Accordingly, in the disclosed recording medium, some of the recording layers are distant from the guide track layer. Also, with the disclosed configuration, since servo control is performed by detecting reflected light from the guide track layer, if the recording medium is tilted in the radial direction in relation to the incidence angle of light, a tracking error may occur on a recording layer that is distant from the guide track layer. For example, on a recording layer that is 1 mm distant from the guide track layer, when the recording medium is tilted 1 degree in relation to the incidence angle of the light, the focal point of the light is shifted as much as 17.4 μm. On a Blu-ray disk with a track pitch of 0.32 μm, 17.4 μm is equivalent to about 50 tracks. For this reason, the recording medium disclosed in patent document 1 requires a high-precision tilt control mechanism when the number of recording layers is large.
Patent document 2 discloses an optical information recording medium where a first guide track layer and a second guide track layer are provided, respectively, above and below a set of recording layers for effective tilt control. The first and second guide track layers have alternate grooves and ridges and the grooves and ridges of the first and second guide track layers are aligned. One disadvantage of the disclosed technology is that it is very difficult to accurately align the grooves and ridges of the first and second layers. In other words, it is very difficult to actually produce the disclosed optical information recording medium.